Multi-piece solid golf ball

ABSTRACT

In a golf ball having a core, a cover and an intermediate layer therebetween, the ball and a sphere consisting of the core encased by the intermediate layer have surface hardnesses which satisfy a specific relationship, the intermediate layer and the cover have thicknesses which satisfy a specific relationship, and (initial velocity of intermediate layer-encased sphere)/(initial velocity of core)≧0.995. Also, the core, the intermediate layer-encased sphere and the ball have respective deflections under given compression conditions which satisfy a specific condition. When used by golfers whose head speed is not very fast, the ball achieves a good distance on shots with a driver, in addition to which it has a soft feel and is able to retain a high spin performance on approach shots.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2015-118026 filed in Japan on Jun. 11, 2015, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a multi-piece solid golf ball of three or more pieces having a core, an intermediate layer and a cover.

BACKGROUND ART

Many golf balls that improve the flight performance on full shots with a driver (W#1) by amateur golfers whose head speed is not very fast have hitherto been disclosed. For example, art which relates to golf balls of two or more pieces having a core and a cover or to multi-piece solid golf balls of three or more pieces having a core, an intermediately layer and a cover, and which is focused on the hardness profile in the core, the hardness relationship between the intermediate layer and the cover, and the intermediate layer material has been disclosed. Such golf balls are described in, for example, JP-A 2014-187351, JP-A 2011-120898, JP-A 2010-214105, JP-A 2010-172702, JP-A 2008-194474 and JP-A 2008-194473.

Yet, there remains room for improvement in achieving an increased distance in such golf balls. To achieve not only an increased distance, but also further increase the enjoyability of the game, it is desired that a high spin performance on approach shots be retained. In addition, it is also desired that the ball have a good durability when repeatedly struck.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a multi-piece solid golf ball which, while maintaining a good distance on shots with a driver (W#1) by an amateur golfer whose head speed is not very fast, is able to retain a high spin performance on approach shots.

As a result of extensive investigations, we have discovered that, in a multi-piece solid golf ball having a core, a cover and an intermediate layer therebetween, by providing the ball with a construction such that it has both a specific relationship between the surface hardness of a sphere composed of the core peripherally encased with the intermediate layer (intermediate layer-encased sphere) and the surface hardness of the ball and also a specific relationship between the thickness of the intermediate layer and the thickness of the cover, such that the initial velocity of the intermediate layer-encased sphere and the initial velocity of the core satisfy the condition (initial velocity of intermediate layer-encased sphere)/(initial velocity of core)≧0.995, and moreover such that, letting the deflections (mm) of the core, intermediate layer-encased sphere and ball when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) be respectively A, B and C, the condition A+B+C≧11.5 (with the proviso that A≧4.0 and C≧3.2) is satisfied, the spin rate of the ball on full shots with a driver (W#1), particularly by an amateur golfer having a head speed of 35 m/s or less, can be kept low, enabling an increased distance to be achieved, and yet the ball is able to retain a high spin performance on approach shots. As a result, we have succeeded in developing a superior golf ball which provides good enjoyability in the game of golf. We have found, moreover, that this golf ball has a soft feel on full shots with a driver (W#1) and a good durability when repeatedly struck. As used herein, “amateur golfer” refers in particular to a player having a head speed (HS) of 35 m/s or less.

Accordingly, the invention provides a multi-piece solid golf having a core, a cover and an intermediate layer therebetween, wherein a sphere composed of the core and the intermediate layer which peripherally encases the core (intermediate layer-encased sphere) and the ball have respective surface hardnesses, expressed in terms of Shore D hardness, which satisfy the relationship:

(Shore D hardness at ball surface)≦(Shore D hardness at surface of intermediate layer-encased sphere);  (1)

the intermediate layer and the cover have respective thicknesses which satisfy the relationship:

cover thickness≦intermediate layer thickness;  (2)

the intermediate layer-encased sphere and the core have respective initial velocities which satisfy the relationship:

(initial velocity of intermediate layer-encased sphere)/(initial velocity of core)≧0.995; and  (3)

the core, the intermediate layer-encased sphere and the ball, when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), have respective deflections (mm) A, B and C which satisfy the condition:

A+B+C≧11.5,  (4)

with the proviso that A≧4.0 and C≧3.2.

In a preferred embodiment, the golf ball, in formula (3), satisfies the condition:

(initial velocity of intermediate layer-encased sphere)/(initial velocity of core)≧1.004.

In another preferred embodiment, the golf ball, in formula (4), satisfies the condition (A−C)≧0.9.

The core preferably has a hardness profile which, expressed in terms of JIS-C hardness, satisfies conditions (i) to (vi) below, wherein Cc is the JIS-C hardness at a center of the core, C5 is the JIS-C hardness at a position 5 mm from the core center, C10 is the JIS-C hardness at a position 10 mm from the core center, C15 is the JIS-C hardness at a position 15 mm from the core center, and Cs is the JIS-C hardness at a surface of the core:

18≦Cs−Cc,  (i)

0<C10−Cc≦10,  (ii)

C10−Cc<Cs−C10,  (iii)

10<Cs−C10,  (iv)

Cs≧68, and  (v)

Cc≧48.  (vi)

In another preferred embodiment, the golf ball further satisfies condition (iii-a) below:

(Cs−C10)/(C10−Cc)≧1.0.  (iii-a)

In yet another preferred embodiment, the golf ball further satisfies condition (vii) below:

Cs−Cc<10.  (vii)

The intermediate layer is preferably formed of a material obtained by blending as essential components: 100 parts by weight of a resin component comprising, in admixture,

-   -   a base resin of (a) an olefin-unsaturated carboxylic acid random         copolymer and/or a metal ion neutralization product of an         olefin-unsaturated carboxylic acid random copolymer mixed         with (b) an olefin-unsaturated carboxylic acid-unsaturated         carboxylic acid ester random terpolymer and/or a metal ion         neutralization product of an olefin-unsaturated carboxylic         acid-unsaturated carboxylic acid ester random terpolymer in a         weight ratio between 100:0 and 0:100, and     -   (e) a non-ionomeric thermoplastic elastomer in a weight ratio         between 100:0 and 50:50;         (c) 5 to 80 parts by weight of a fatty acid and/or fatty acid         derivative having a molecular weight of from 228 to 1,500; and         (d) 0.1 to 17 parts by weight of a basic inorganic metal         compound capable of neutralizing un-neutralized acid groups in         the base resin and component (c).

In a further preferred embodiment, the ball, the intermediate layer-encased sphere and the core have respective initial velocities which satisfy the relationship:

initial velocity of ball<initial velocity of core<initial velocity of envelope layer-encased sphere.  (5)

In a still further preferred embodiment, the golf ball, in formula (1), satisfies the condition:

(Shore D hardness at surface of intermediate layer-encased sphere)≧(Shore D hardness at core surface).  (1′)

Advantageous Effects of the Invention

The multi-piece solid golf ball of the invention, when played by a golfer whose head speed is not very fast (particularly a golfer having a head speed of 35 m/s or less), is able to achieve a good distance on shots with a driver (W#1) and also can provide a soft feel. Moreover, this golf ball is able to retain a high spin performance on approach shots and has a good durability to repeated impact.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic sectional diagram of a golf ball according to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description, taken in conjunction with the foregoing diagram.

The multi-piece solid golf ball of the invention has, in order from the inside: a solid core, an intermediate layer, and a cover. Referring to FIG. 1, which shows the internal structure of one embodiment of the inventive golf ball, the golf ball G has a core 1, an intermediate layer 2 encasing the core 1, and a cover 3 encasing the intermediate layer 2. Numerous dimples D are typically formed on the surface of the cover 3 to improve the aerodynamic properties of the ball. The respective layers are described in detail below.

The core diameter, although not particularly limited, is generally from 34.9 to 40.3 mm, preferably from 36.1 to 39.4 mm, and more preferably from 37.3 to 38.5 mm. When the core diameter is too small, the spin rate on shots with a driver (W#1) may rise, as a result of which the intended distance may not be obtained. When the core diameter is too large, the durability to cracking on repeated impact may worsen, or the feel of the ball at impact may worsen.

The core deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), although not particularly limited, is preferably from 4.0 to 5.3 mm, more preferably from 4.1 to 5.1 mm, and even more preferably from 4.3 to 4.9 mm. When this value is too high, the feel of the ball at impact may be too soft and the durability on repeated impact may worsen, or the initial velocity on full shots may decrease, as a result of which the intended distance may not be obtained. On the other hand, when this value is too low, the feel of the ball becomes too hard and the spin rate on full shots rises, as a result of which the intended distance is not achieved.

Next, in the core hardness profile, the specific hardness range differs for cases where the hardness difference between the surface and center of the core, expressed in terms of JIS-C hardness, is 15 or more and for cases where this hardness difference is 10 or less. The details are as follows.

I. JIS-C Hardness Difference Between Core Surface and Center ≧15

This core hardness profile has a hardness state with no gradient from the core center to a specific cross-sectional position but with a steep gradient from a specific cross-sectional position to the surface, and is able, in particular, to fully achieve a spin rate-lowering effect and thus improve the flight performance. The hardnesses at specific positions of the core interior are explained below.

The core surface hardness (Cs), expressed in terms of JIS-C hardness, is preferably from 68 to 80, more preferably from 70 to 78, and even more preferably from 72 to 76. When the JIS-C hardness value for the core surface hardness is too large, the feel at impact may harden or the durability of the ball to cracking on repeated impact may worsen. On the other hand, when this value is too small, the spin rate may rise excessively and the rebound may decrease, as a result of which the ball may not achieve a good distance.

The core center hardness (Cc), expressed in terms of JIS-C hardness, is preferably from 48 to 62, more preferably from 51 to 60, and even more preferably from 53 to 58. When the JIS-C hardness value for the core center hardness is too large, the spin rate may rise excessively, as a result of which a good distance may not be obtained, and the ball may have a hard feel at impact. On the other hand, when this value is too small, the durability to cracking on repeated impact may worsen and the ball may have too soft a feel at impact.

The JIS-C hardness at a position 5 mm from the core center (C5) is preferably from 52 to 62, more preferably from 54 to 60, and even more preferably from 56 to 68. The JIS-C hardness at a position 10 mm from the core center (C10) is preferably from 56 to 67, more preferably from 58 to 65, and even more preferably from 60 to 63. When these hardness values are too large, the spin rate may rise excessively, as a result of which a good distance may not be obtained, and the ball may have a hard feel at impact. On the other hand, when these values are too small, the durability to cracking on repeated impact may worsen and the ball may have too soft a feel at impact.

The JIS-C hardness at a position 15 mm from the core center (C15) is preferably from 64 to 78, more preferably from 66 to 76, and even more preferably from 68 to 74. When this hardness value is too large, the ball may have a hard feel and the durability to cracking on repeated impact may worsen. On the other hand, when this hardness value is too small, the spin rate may rise excessively and the rebound may decrease, as a result of which a good distance may not be obtained.

The Cs−C15 value is preferably from 1 to 9, more preferably from 2 to 7, and even more preferably from 3 to 5. When this value is too large, the durability to cracking on repeated impact may worsen. On the other hand, when this value is too small, the spin rate may rise excessively, as a result of which a good distance may not be obtained.

The C15−C10 value is preferably from 4 to 15, more preferably from 6 to 13, and even more preferably from 8 to 11. When this value is too large, the durability to cracking on repeated impact may worsen. On the other hand, when this value is too small, the spin rate may rise excessively, as a result of which a good distance may not be obtained.

The C10−C5 value is preferably from 1 to 7, more preferably from 2 to 5, and even more preferably from 3 to 4. When this value falls outside of the above range, the spin rate on full shots may rise excessively, as a result of which a good distance may not be obtained. Also, the durability to cracking on repeated impact may worsen.

The C5−Cc value is preferably from 0 to 7, more preferably from 1 to 5, and even more preferably from 2 to 3. When this value is too large, the spin rate may rise excessively, as a result of which a good distance may not be obtained.

The C10−Cc value is preferably more than 0 and up to 10, and more preferably from 2 to 8, meaning that the hardness gradient from the core center (Cc) to a position 10 mm from the core center (C10) is not very steep. When this value is too large, the spin rate on full shots may rise excessively, as a result of which a good distance may not be obtained.

The Cs−C10 value is preferably at least 10, and more preferably from 11 to 15, meaning that the hardness gradient from a position 10 mm from the core center (C10) out to the core surface (Cs) is steep to a degree that exceeds a JIS-C hardness of 10. When this value is too large, the durability to cracking on repeated impact may worsen. On the other hand, when this value is too small, the spin rate on full shots may rise excessively, as a result of which a good distance may not be obtained.

It is critical for the Cs−C10 value to be larger than the C10−Cc value, meaning that the hardness gradient is steeper in the outer portion of the core than in the inner portion of the core. That is, the value (Cs−C10)/(C10−Cc) is preferably from 1.0 to 5.0, more preferably from 1.2 to 4.0, and even more preferably from 1.5 to 3.0. When this value is too large, the durability to cracking on repeated impact may worsen. On the other hand, when this value is too small, the spin rate may rise excessively, as a result of which a good distance may not be obtained.

The hardness difference between the surface and center of the core, i.e., the Cs−Cc value, is preferably from 15 to 30, more preferably from 16 to 24, and even more preferably from 18 to 20. When this hardness difference is too large, the durability to cracking on repeated impact may worsen. On the other hand, when the hardness difference is too small, the spin rate may rise excessively, as a result of which a good distance may not be obtained.

II. JIS-C Hardness Difference Between Core Surface and Center ≦10

This core hardness profile is a profile that is nearly flat with little gradient from the core surface to the core center. The hardnesses at specific positions of the core interior are explained below.

The core surface hardness (Cs), expressed in terms of JIS-C hardness, is preferably from 59 to 73, more preferably from 61 to 71, and even more preferably from 63 to 69. When the JIS-C hardness value for this core surface hardness is too large, the feel at impact may harden or the durability of the ball to cracking on repeated impact may worsen. On the other hand, when this value is too small, the spin rate may rise excessively and the rebound may decrease, as a result of which the ball may not achieve a good distance.

The core center hardness (Cc), expressed in terms of JIS-C hardness, is preferably from 56 to 68, more preferably from 58 to 66, and even more preferably from 60 to 64. When the JIS-C hardness value for this core center hardness is too large, the spin rate may rise excessively, as a result of which a good distance may not be obtained, and the ball may have a hard feel at impact. On the other hand, when this value is too small, the durability to cracking on repeated impact may worsen and the ball may have too soft a feel at impact.

The JIS-C hardness at a position 5 mm from the core center (C5) is preferably from 56 to 68, more preferably from 58 to 66, and even more preferably from 60 to 64. The JIS-C hardness at a position 10 mm from the core center (C10) is preferably from 56 to 68, more preferably from 58 to 66, and even more preferably from 60 to 64. When these hardness values are too large, the spin rate may rise excessively, as a result of which a good distance may not be obtained, and the ball may have a hard feel at impact. On the other hand, when these values are too small, the rebound may decrease, as a result of which a good distance may not be obtained, and the feel at impact may be too soft.

The JIS-C hardness at a position 15 mm from the core center (C15) is preferably from 57 to 69, more preferably from 59 to 67, and even more preferably from 61 to 65. When this hardness value is too large, the ball may have a hard feel and the durability to cracking on repeated impact may worsen. On the other hand, when this hardness value is too small, the spin rate may rise excessively and the rebound may decrease, as a result of which a good distance may not be obtained.

The Cs−C15 value is preferably not more than 9, more preferably from 0 to 7, and even more preferably from 1 to 5. When this value is too large, the durability to cracking on repeated impact may worsen. On the other hand, when this value is too small, the spin rate may rise, as a result of which a good distance may not be obtained.

The C15−C10 value is preferably from 0 to ±3, more preferably from 0 to ±2, and even more preferably from 0 to ±1. When this value falls outside of this range, a good durability to cracking on repeated impact may not be obtained.

The C10−C5 value is preferably from 0 to ±3, more preferably from 0 to ±2, and even more preferably from 0 to ±1. When this value falls outside of the above range, a good durability to cracking on repeated impact may not be obtained.

The C5−Cc value is preferably from 0 to ±3, more preferably from 0 to ±2, and even more preferably from 0 to ±1. When this value falls outside of the above range, a good durability to cracking on repeated impact may not be obtained.

The hardness difference between the core surface and the core center, i.e., the Cs−C10 value, is preferably from 0 to 10, more preferably from 1 to 8, and even more preferably from 2 to 6. When this hardness difference value is too large, the durability to cracking on repeated impact may worsen. On the other hand, when this hardness difference value is too small, the spin rate on full shots may rise excessively, as a result of which a good distance may not be obtained.

The center hardness (Cc) and cross-sectional hardnesses at specific positions refer to the hardnesses measured at the center and specific positions on a cross-section obtained by cutting a golf ball core in half through the center. The surface hardness (Cs) refers to the hardness measured at the spherical surface of the core.

The core having the above hardness profile and deflection is preferably made of a material that is composed primarily of rubber. For example, use may be made of a rubber composition obtained by compounding (A) a base rubber as the chief component, (B) an organic peroxide, and also a co-crosslinking agent, an inert filler and, optionally, an organosulfur compound.

Polybutadiene is preferably used as the base rubber (A). The polybutadiene has a cis-1,4 bond content on the polymer chain of typically at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. When the content of cis-1,4 bonds among the bonds on the polybutadiene molecule is too low, the resilience may decrease.

Rubber components other than this polybutadiene may be included in the base rubber (A) within a range that does not detract from the advantageous effects of the invention. Examples of such rubber components other than the foregoing polybutadiene include other polybutadienes, and diene rubbers other than polybutadiene, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.

The organic peroxide (B) used in the invention is not particularly limited, although the use of an organic peroxide having a one-minute half-life temperature of 110 to 185° C. is preferred. One, two or more organic peroxides may be used. The amount of organic peroxide included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, and more preferably at least 0.3 part by weight. The upper limit is preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, and even more preferably not more than 3 parts by weight. A commercially available product may be used as the organic peroxide. Specific examples include those available under the trade names Percumyl D, Perhexa C-40, Niper BW and Peroyl L (all from NOF Corporation), and Luperco 231XL (from Atochem Co.).

The co-crosslinking agent is exemplified by unsaturated carboxylic acids and the metal salts of unsaturated carboxylic acids. Illustrative examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred. Metal salts of unsaturated carboxylic acids are not particularly limited, and are exemplified by those obtained by neutralizing the foregoing unsaturated carboxylic acids with the desired metal ions. Illustrative examples include the zinc salts and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

These unsaturated carboxylic acids and/or metal salts thereof are included in an amount per 100 parts by weight of the base rubber which is typically at least 10 parts by weight, preferably at least 15 parts by weight, and more preferably at least 20 parts by weight. The upper limit is typically not more than 60 parts by weight, preferably not more than 50 parts by weight, more preferably not more than 45 parts by weight, and most preferably not more than 40 parts by weight. When too much is included, the feel of the ball may become too hard and unpleasant. When too little is included, the rebound may decrease.

To satisfy the desired hardness profile described above, the core is preferably formed of a material molded under heat from a rubber composition which includes, as the essential ingredients: (A) a base rubber, (B) an organic peroxide, and (C) water and/or a metal monocarboxylate.

Decomposition of the organic peroxide within the core formulation can be promoted by the direct addition of water (or a water-containing material) to the core material. It is known that the decomposition efficiency of the organic peroxide within the core-forming rubber composition changes with temperature and that, starting at a given temperature, the decomposition efficiency rises with increasing temperature. If the temperature is too high, the amount of decomposed radicals rises excessively, leading to recombination between radicals and, ultimately, deactivation. As a result, fewer radicals act effectively in crosslinking. Here, when a heat of decomposition is generated by decomposition of the organic peroxide at the time of core vulcanization, the vicinity of the core surface remains at substantially the same temperature as the temperature of the vulcanization mold, but the temperature near the core center, due to the build-up of heat of decomposition by the organic peroxide which has decomposed from the outside, becomes considerably higher than the mold temperature. In cases where water (or a water-containing material) is added directly to the core, because the water acts to promote decomposition of the organic peroxide, radical reactions like those described above can be made to differ at the core center and at the core surface. That is, decomposition of the organic peroxide is further promoted near the center of the core, bringing about greater radical deactivation, which leads to a further decrease in the amount of active radicals. As a result, it is possible to obtain a core in which the crosslink densities at the core center and the core surface differ markedly. It is also possible to obtain a core having different dynamic viscoelastic properties at the core center. Along with achieving a lower spin rate, golf balls having such a core are also able to exhibit excellent durability and undergo less change over time in rebound. When zinc monoacrylate is used instead of the above water, water is generated from the zinc monoacrylate by heat during kneading of the compounding materials. An effect similar to that obtained by the addition of water can thereby be obtained.

Components A and B have already been described above.

The water serving as component C is not particularly limited, and may be distilled water or tap water. The use of distilled water which is free of impurities is especially preferred. The amount of water included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, and more preferably at least 0.3 part by weight. The upper limit is preferably not more than 5 parts by weight, and more preferably not more than 4 parts by weight.

By including a suitable amount of such water, the moisture content in the rubber composition prior to vulcanization becomes preferably at least 1,000 ppm, and more preferably at least 1,500 ppm. The upper limit is preferably not more than 8,500 ppm, and more preferably not more than 8,000 ppm. When the moisture content of the rubber composition is too low, it may be difficult to obtain a suitable crosslink density and tan δ, which may make it difficult to mold a golf ball having little energy loss and a reduced spin rate. On the other hand, when the moisture content of the rubber composition is too high, the core may end up too soft, which may make it difficult to obtain a suitable core initial velocity.

It is also possible to add water directly to the rubber composition. The following methods (i) to (iii) may be employed to include water:

-   (i) applying steam or ultrasonically applying water in the form of a     mist to some or all of the rubber composition (compounded material); -   (ii) immersing some or all of the rubber composition in water; -   (iii) letting some or all of the rubber composition stand for a     given period of time in a high-humidity environment in a place where     the humidity can be controlled, such as a constant humidity chamber.

As used herein, “high-humidity environment” is not particularly limited, so long as it is an environment capable of moistening the rubber composition, although a humidity of from 40 to 100% is preferred.

Alternatively, the water may be worked into a jelly state and added to the above rubber composition. Or a material obtained by first supporting water on a filler, unvulcanized rubber, rubber powder or the like may be added to the rubber composition. In such a form, the workability is better than when water is added directly to the composition, enabling the efficiency of golf ball production to be increased. The type of material in which a given amount of water has been included, although not particularly limited, is exemplified by fillers, unvulcanized rubbers and rubber powders in which sufficient water has been included. The use of a material which causes no loss of durability or resilience is especially preferred. The moisture content of the above material is preferably at least 3 wt %, more preferably at least 5 wt %, and even more preferably at least 10 wt %. The upper limit is preferably not more than 99 wt %, and even more preferably not more than 95 wt %.

In this invention, a metal monocarboxylate may be used instead of the above-described water. Metal monocarboxylates, in which the carboxylic acid is presumably coordination-bonded to the metal, are distinct from metal dicarboxylates such as zinc diacrylate of the formula (CH₂═CHCOO)₂Zn. A metal monocarboxylate introduces water into the rubber composition by way of a dehydration/condensation reaction, and thus provides an effect similar to that of water. Moreover, because a metal monocarboxylate can be added to the rubber composition as a powder, the operations can be simplified and uniform dispersion within the rubber composition is easy. A monosalt is required in order to carry out the above reaction effectively. The amount of metal monocarboxylate included per 100 parts by weight of the base rubber is preferably at least 1 part by weight, and more preferably at least 3 parts by weight. The upper limit in the amount of metal monocarboxylate included is preferably not more than 60 parts by weight, and more preferably not more than 50 parts by weight. When the amount of metal monocarboxylate included is too small, it may be difficult to obtain a suitable crosslink density and tan δ, as a result of which a sufficient golf ball spin rate-lowering effect may not be achievable. On the other hand, when too much is included, the core may become too hard, as a result of which it may be difficult for the ball to maintain a suitable feel at impact.

The carboxylic acid used may be, for example, acrylic acid, methacrylic acid, maleic acid, fumaric acid or stearic acid. Examples of the substituting metal include sodium, potassium, lithium, zinc, copper, magnesium, calcium, cobalt, nickel and lead, although the use of zinc is preferred. Illustrative examples of the metal monocarboxylate include zinc monoacrylate and zinc monomethacrylate, with the use of zinc monoacrylate being especially preferred.

Core production may be carried out in the usual manner by molding a spherical molded article (core) using heat and compression under vulcanization conditions of at least 140° C. and not more than 180° C. and at least 10 minutes and not more than 60 minutes.

The vulcanized core preferably has a higher moisture content at the core center than at the core surface. The moisture content of the molded core can be suitably controlled by adjusting such conditions as the amount of water included in the rubber composition, the molding temperature and the molding time.

Next, the intermediate layer is described. The intermediate layer has a material hardness expressed in terms of Shore D hardness which, although not particularly limited, is preferably from 48 to 68, more preferably from 52 to 62, and even more preferably from 55 to 57. The sphere encased by the intermediate layer (referred to below as the “intermediate layer-encased sphere”) has a surface hardness, expressed in terms of Shore D hardness, which is preferably from 55 to 75, more preferably from 59 to 69, and even more preferably from 62 to 64. When the intermediate layer is too soft, the spin rate on full shots may rise excessively, as a result of which a good distance may not be achieved. On the other hand, when the intermediate layer is too hard, the durability to cracking on repeated impact may worsen and the feel of the ball on shots with a putter or on short approaches may worsen.

The intermediate layer-encased sphere has a deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably from 3.2 to 4.6 mm, more preferably from 3.4 to 4.4 mm, and even more preferably from 3.6 to 4.2 mm. When this value is too high, the feel of the ball may be too soft, the durability to repeated impact may be poor, and the initial velocity on full shots may be low, as a result of which the intended distance may not be achieved. On the other hand, when this value is too low, the feel of the ball may be too hard and the spin rate on full shots may rise, as a result of which the intended distance may not be achieved.

The intermediate layer has a thickness of preferably from 0.9 to 2.4 mm, more preferably from 1.2 to 2.1 mm, and even more preferably from 1.5 to 1.8 mm. It is preferable for the thickness of the intermediate layer to be higher than that of the subsequently described cover (outermost layer). When the intermediate layer thickness falls outside of this range or is thinner than the cover, the spin rate-reducing effects on shots with a driver (W#1) may be inadequate, as a result of which a good distance may not be achieved.

The intermediate layer material is not particularly limited, although preferred use can be made of various thermoplastic resin materials. To fully achieve the desired effects of the invention, it is especially preferable to use a high-resilience resin material as the intermediate layer material. For example, the use of an ionomer resin material or the subsequently described highly neutralized resin material is preferred.

By way of illustration, preferred use can be made of, as the intermediate layer material, a material containing as the essential component a base resin of, mixed in specific amounts: (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer, and (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer.

Commercially available products may be used as components (a) and (b). Illustrative examples of the random copolymer in component (a) include Nucrel® N1560, N1214, N1035 and AN4221C (all products of DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of the random copolymer in component (b) include Nucrel® AN4311, AN4318 and AN4319 (all products of DuPont-Mitsui Polychemicals Co., Ltd.).

Illustrative examples of the metal ion neutralization product of the random copolymer in component (a) include Himilan® 1554, 1557, 1601, 1605, 1706 and AM7311 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), and Surlyn® 7930 (E.I. DuPont de Nemours & Co.). Illustrative examples of the metal ion neutralization product of the random copolymer in component (b) include Himilan® 1855, 1856 and AM7316 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), and Surlyn® 6320, 8320, 9320 and 8120 (all products of E.I. DuPont de Nemours & Co.). Sodium-neutralized ionomer resins that are suitable as the metal ion neutralization product of the random copolymer include Himilan® 1605, 1601 and 1555.

When preparing the base resin, the weight ratio in which component (a) and component (b) are mixed is set to generally between 100:0 and 0:100. The ratio of component (a) with respect to the combined amount of components (a) and (b) may be set to preferably at least 50% by weight, more preferably at least 75% by weight, and most preferably 100% by weight.

A non-ionomeric thermoplastic elastomer (e) may be added to the base resin in order to enhance even further the feel of the ball at impact and the ball rebound. Examples of component (e) include olefin elastomers, styrene elastomers, polyester elastomers, urethane elastomers and polyamide elastomers. To further increase the rebound, it is preferable to use a polyester elastomer or an olefin elastomer in this invention. The use of an olefin elastomer consisting of a thermoplastic block copolymer which includes crystalline polyethylene blocks as the hard segments is especially preferred.

A commercially available product may be used as component (e). Illustrative examples include Dynaron (JSR Corporation) and the polyester elastomer Hytrel® (DuPont-Toray Co., Ltd.).

The content of component (e) must be set to more than 0 parts by weight. The upper limit may be set to preferably not more than 100 parts by weight, more preferably not more than 60 parts by weight, even more preferably not more than 50 parts by weight, and most preferably not more than 40 parts by weight, per 100 parts by weight of the base resin. When the component (e) content is too high, the compatibility of the mixture may decrease and the durability of the golf ball may markedly decline.

A fatty acid or fatty acid derivative having a molecular weight of at least 228 and not more than 1,500 may be added to the base resin as component (c). Compared with the base resin, this component (c) has a very low molecular weight and, by suitably adjusting the melt viscosity of the mixture, helps in particular to improve the flow properties. Component (c) includes a relatively high content of acid groups (or derivatives thereof), and can suppress an excessive loss of resilience.

The amount of component (c) included per 100 parts by weight of the resin component suitably composed of components (a), (b) and (e) may be set to at least 5 parts by weight, preferably at least 10 parts by weight, more preferably at least 15 parts by weight, and even more preferably at least 18 parts by weight. The upper limit in the amount of component (c) may be set to not more than 80 parts by weight, preferably not more than 70 parts by weight, more preferably not more than 60 parts by weight, and even more preferably not more than 50 parts by weight. When the amount of component (c) included is too small, the melt viscosity may decrease, lowering the processability; when the amount included is too large, the durability may decrease.

A basic inorganic metal compound capable of neutralizing acid groups in the base resin and component (c) may be added as component (d). By including component (d), the acid groups present in the base resin and component (c) are neutralized and, owing to synergistic effects from blending these components, the thermal stability of the resin composition increases. At the same time, a good moldability is imparted, enabling the resilience of the molded product to be enhanced.

The amount of component (d) included per 100 parts by weight of the above resin component must be at least 0.1 part by weight, and may be set to preferably at least 0.5 part by weight, more preferably at least 1 part by weight, and even more preferably at least 2 parts by weight. The upper limit may be set to not more than 17 parts by weight, preferably not more than 15 parts by weight, more preferably not more than 13 parts by weight, and even more preferably not more than 10 parts by weight. Including too little component (d) fails to improve thermal stability and resilience, whereas including too much lowers the heat resistance of the golf ball material owing to the presence of excess basic inorganic metal compound.

As mentioned above, by including specific amounts of components (c) and (d) with respect to the resin component composed of a base resin of specific amounts of components (a) and (b) in admixture with optional component (e), the resin material can be endowed with an excellent thermal stability, flowability and moldability, and the resulting molded product can be endowed with a dramatically improved resilience.

It is recommended that the material formulated from the resin component and components (c) and (d) have a high degree of neutralization (i.e., that it be highly neutralized). Specifically, it is recommended that at least 50 mol %, preferably at least 60 mol %, more preferably at least 70 mol %, and even more preferably at least 80 mol %, of the acid groups within the material be neutralized. Such high neutralization of acid groups in the material makes it possible to more reliably suppress the exchange reactions that cause trouble when only a base resin and a fatty acid (or fatty acid derivative) are used as in the above-cited prior art, thus preventing the generation of fatty acid. As a result, the thermal stability is greatly improved and the moldability is good, enabling molded products to be obtained which have an excellent resilience compared with prior-art ionomer resins.

Here, “degree of neutralization” refers to the degree of neutralization of acid groups present within the mixture of the base resin and the fatty acid (or fatty acid derivative) serving as component (c), and differs from the degree of neutralization of the ionomer resin itself when an ionomer resin is used as the metal ion neutralization product of a random copolymer in the base resin. On comparing such a mixture having a certain degree of neutralization with an ionomer resin alone having the same degree of neutralization, the mixture of the invention, by including component (d), contains a very large number of metal ions and thus has a higher density of ionic crosslinks which contribute to improved resilience, making it possible to confer the molded product with an excellent resilience.

Optional additives may be suitably included in the intermediate layer material according to the intended use. For example, various additives such as pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added. When such additives are included, the amount thereof, per 100 parts by weight of components (a) to (e) combined, is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight, with the upper limit being preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.

It is advantageous to abrade the surface of the intermediate layer in order to increase adhesion of the intermediate layer material with the polyurethane that is preferably used in the subsequently described cover (outermost layer). In addition, it is desirable to apply a primer (adhesive) to the surface of the intermediate layer following such abrasion treatment or to add an adhesion reinforcing agent to the intermediate layer material.

The intermediate layer material has a specific gravity which is typically less than 1.1, preferably from 0.90 to 1.05, and more preferably from 0.93 to 0.99. Outside of this range, the rebound becomes small, as a result of which a good distance may not be obtained, or the durability to cracking on repeated impact may worsen.

Next, the cover, which is the outermost layer of the ball, is described.

The cover (outermost layer) has a material hardness expressed in terms of Shore D hardness which, although not particularly limited, is preferably from 44 to 58, more preferably from 48 to 56, and even more preferably from 52 to 54.

The cover (outermost layer) encased sphere, i.e., the ball, has a surface hardness, expressed in terms of Shore D hardness, which is preferably from 52 to 67, more preferably from 56 to 65, and even more preferably from 60 to 63. When the cover-encased sphere is too much softer than this range, the spin rate on shots with a driver (W#1) and on iron shots may become too high, as a result of which a good distance may not be obtained. When the surface hardness is higher than this range, the spin rate on approach shots may be inadequate or the feel at impact may be too hard.

The cover (outermost layer) encased sphere, that is, the ball, has a deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably from 3.2 to 4.1 mm, more preferably from 3.3 to 3.9 mm, and even more preferably from 3.4 to 3.7 mm. When this value is too high, the feel of the ball may be too soft, the durability to repeated impact may worsen, or the initial velocity on full shots may be low, as a result of which the intended distance may not be achieved. On the other hand, when this value is too low, the feel of the ball may be too hard and the spin rate on full shots may rise, as a result of which the intended distance may not be achieved.

The cover (outermost layer) has a thickness which, although not particularly limited, is preferably from 0.3 to 1.5 mm, more preferably from 0.45 to 1.2 mm, and even more preferably from 0.6 to 0.9 mm. When the cover is thicker than this range, the rebound on W#1 shots and iron shots may be inadequate and the spin rate may rise, as a result of which a good distance may not be obtained. On the other hand, when the cover is thinner than this range, the scuff resistance may worsen and the ball may lack spin receptivity on approach shots, resulting in poor controllability.

The cover (outermost layer) material is not particularly limited, although the use of any of various types of thermoplastic resin materials is preferred. For reasons having to do with controllability and scuff resistance, it is preferable to use a urethane resin as the cover material of the invention. In particular, from the standpoint of the mass productivity of manufactured golf balls, it is preferable to use a cover material composed primarily of a thermoplastic polyurethane, with formation more preferably being carried out using a resin blend composed primarily of (O) a thermoplastic polyurethane and (P) a polyisocyanate compound.

In the thermoplastic polyurethane composition containing above components (O) and (P), to improve the ball properties even further, a necessary and sufficient amount of unreacted isocyanate groups should be present in the cover resin material. Specifically, it is recommended that the combined weight of above components (O) and (P) be at least 60%, and more preferably at least 70%, of the weight of the overall cover layer. Components (O) and (P) are described below in detail.

The thermoplastic polyurethane (O) has a structure which includes soft segments consisting of a polymeric polyol (polymeric glycol) that is a long-chain polyol, and hard segments consisting of a chain extender and a polyisocyanate compound. Here, the long-chain polyol serving as a starting material may be any that has hitherto been used in the art relating to thermoplastic polyurethanes, and is not particularly limited. Illustrative examples include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly, or two or more may be used in combination. Of these, in terms of being able to synthesize a thermoplastic polyurethane having a high rebound resilience and excellent low-temperature properties, a polyether polyol is preferred.

Any chain extender that has hitherto been employed in the art relating to thermoplastic polyurethanes may be advantageously used as the chain extender. For example, low-molecular-weight compounds with a molecular weight of 400 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups are preferred. Examples of the chain extender include, but are not limited to, 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Of these, an aliphatic diol having 2 to 12 carbons is preferred, and 1,4-butylene glycol is more preferred, as the chain extender.

Any polyisocyanate compound hitherto employed in the art relating to thermoplastic polyurethanes may be advantageously used without particular limitation as the polyisocyanate compound. For example, use may be made of one, two or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. However, depending on the type of isocyanate, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use the following aromatic diisocyanate: 4,4′-diphenylmethane diisocyanate.

Commercially available products may be used as the thermoplastic polyurethane serving as component (O). Illustrative examples include Pandex T-8295, T-8290, T-8283 and T-8260 (all from DIC Bayer Polymer, Ltd.).

Although not an essential ingredient, a thermoplastic elastomer other than the above thermoplastic polyurethane may be included as an additional component together with above components (O) and (P). By including this component (Q) in the above resin blend, a further improvement in the flowability of the resin blend can be achieved and the properties required of a golf ball cover material, such as resilience and scuff resistance, can be enhanced.

The relative proportions of above components (O), (P) and (Q) are not particularly limited. However, to fully elicit the desirable effects of the invention, the weight ratio (O):(P):(Q) is preferably from 100:2:50 to 100:50:0, and more preferably from 100:2:50 to 100:30:8.

In addition to the ingredients making up the thermoplastic polyurethane, various additives may be optionally included in the above resin blend. For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and internal mold lubricants may be suitably included.

The manufacture of multi-piece solid golf balls in which the above-described core, intermediate layer and cover (outermost layer) are formed as successive layers may be carried out by a customary method such as a known injection-molding process. For example, a multi-piece golf ball may be obtained by placing a molded and vulcanized product composed primarily of a rubber material as the core in a given injection mold, injecting an intermediate layer material over the core to give an intermediate sphere, and subsequently placing the resulting sphere in another injection mold and injection-molding a cover (outermost layer) material over the sphere. Alternatively, a cover may be formed over the intermediate layer by a method that involves encasing the intermediate sphere with a cover (outermost layer), this being carried out by, for example, enclosing the intermediate sphere within two half-cups that have been pre-molded into hemispherical shapes, and then molding under applied heat and pressure.

The golf ball of the invention preferably satisfies the following conditions.

(1) Relationship Between Deflections Under Specific Loading of Core and Ball

The relationship between the deflections of the core and the ball under specific loading is optimized within a specific range. That is, letting A be the deflection of the core when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and C be the deflection of the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), the value A−C is preferably from 0.7 to 1.5, more preferably from 0.9 to 1.3, and even more preferably from 1.0 to 1.2. When this value is too large, the durability to cracking on repeated impact may worsen, or the feel of the ball on full shots may be too soft. On the other hand, when this value is too small, the spin rate on full shots may become too high, as a result of which the intended distance may not be obtained.

(2) Relationship Between Thicknesses of Intermediate Layer and Cover

The relative thicknesses of the intermediate layer and the cover are set in a specific range. The value obtained by subtracting the cover thickness from the intermediate layer thickness is preferably from 0 to 2.0 mm, more preferably from 0.1 to 1.5 mm, and even more preferably from 0.3 to 1.0 mm. When this value is too large, the feel at impact may become too hard or the core may become too soft, resulting in a poor durability to cracking on repeated impact. On the other hand, when this value is too small, the spin rate on full shots may become too high, as a result of which the intended distance may not be obtained.

Also, the sum of the intermediate layer thickness and the cover thickness is preferably from 1.6 to 3.0 mm, more preferably from 1.8 to 2.8 mm, and even more preferably from 2.0 to 2.6 mm. When this combined thickness is too large, the initial velocity may decrease and the ball may not achieve a good distance on shots with a driver (W#1). On the other hand, when this value is too small, the durability on repeated impact may worsen.

(3) Relationship Between Surface Hardnesses of Ball and Intermediate Layer-Encased Sphere

In order for the ball to have a structure in which the cover is hard on the inside and soft on the outside and the intermediate layer is hard, it is critical for the surface hardnesses of the ball and the intermediate layer-encased sphere to satisfy the relationship:

surface hardness of ball≦surface hardness of intermediate layer-encased sphere.

The value obtained by subtracting the surface hardness of the intermediate layer-encased sphere from the surface hardness of the ball, expressed in terms of Shore D hardness, is preferably from −20 to 0, more preferably from −15 to −1, and even more preferably from −10 to −2. When this value is too large, the spin rate on full shots may rise excessively, as a result of which the intended distance may not be obtained, or the cover may become hard, giving the ball an inadequate spin rate in the short game, as a result of which the controllability may be poor. On the other hand, when this value is too small, the cover may become too soft, leading to excessive spin on full shots, or the initial velocity may be too low, as a result of which the intended distance may not be achieved.

(4) Relationship Between Surface Hardnesses of Core and Ball

The relationship between the surface hardness of the core and the surface hardness of the ball is optimized within a specific range. That is, the value obtained by subtracting the surface hardness of the ball from the surface hardness of the core, expressed in terms of Shore D hardness, is preferably from −22 to 0, more preferably from −18 to −5, and even more preferably from −15 to −10. When this value is too large, the cover may be too hard, making the ball poorly suited for the short game, or the core may be soft, which may result in a poor durability to cracking on repeated impact. On the other hand, when this value is too small, the spin rate on full shots may rise excessively, as a result of which the intended distance may not be obtained.

(5) Relationship Between Deflections Under Specific Loading of Core and Intermediate Layer-Encased Sphere

Letting A be the deflection of the core when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and B be the deflection of the intermediate layer-encased sphere when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), the value A−B is preferably from 0.3 to 1.4, more preferably from 0.5 to 1.2, and even more preferably from 0.6 to 1.0. When this value is too large, the durability to cracking on repeated impact may worsen, or the initial velocity of the ball on full shots may decrease, as a result of which the intended distance may not be obtained. On the other hand, when this value is too small, the spin rate on full shots may become too high, as a result of which the intended distance may not be obtained.

(6) Sum of Deflections of Core. Intermediate Layer-Encased Sphere and Ball

Letting the deflection (mm) of the core, the intermediate layer-encased sphere and the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) be respectively A, B and C, the sum A+B+C is at least 11.5 (with A being at least 4.0 and C being at least 3.2), preferably from 11.5 to 13.0, more preferably from 11.7 to 12.8, and even more preferably from 11.9 to 12.5. When this value is too large, the durability to cracking on repeated impact may worsen and the initial velocity on full shots may decrease, as a result of which the intended distance may not be obtained. On the other hand, when this value is too small, the spin rate on full shots becomes too high, as a result of which the intended distance is not obtained, or the feel of the ball at impact becomes too hard.

(7) Relationship Between Surface Hardnesses of Intermediate Layer-Encased Sphere and Core

The relationship between the surface hardness of the intermediate layer-encased sphere and the surface hardness of the core is optimized within a specific range. That is, the value obtained by subtracting the surface hardness of the core from the surface hardness of the intermediate layer-encased sphere, expressed in terms of Shore D hardness, is preferably from 7 to 24, more preferably from 10 to 20, and even more preferably from 13 to 16. When this value is too large, the durability to cracking under repeated impact may worsen, or the feel at impact may worsen. On the other hand, when this value is too small, the spin rate on full shots may be too high, as a result of which the intended distance may not be obtained.

(8) Relationship Between Initial Velocities of Intermediate Layer-Encased Sphere and Core

The relationship between the initial velocity of the intermediate layer-encased sphere and the initial velocity of the core is optimized within a specific range. That is, the value obtained by subtracting the initial velocity of the core from the initial velocity of the intermediate layer-encased sphere is preferably from 0 to 0.8 m/s, more preferably from 0.1 to 0.6 m/s, and even more preferably from 0.2 to 0.4 m/s. When this value is too large, the initial velocity of the finished ball may not conform to the standard set by The Royal and Ancient Golf Club of St. Andrews (R&A), which may make the ball unacceptable as an official ball. On the other hand, when this value is too small, the spin rate on shots with a driver (W#1) may rise, as a result of which the intended distance may not be achieved.

Also, the value obtained by dividing the initial velocity of the intermediate layer-encased sphere by the initial velocity of the core, or (initial velocity of intermediate layer-encased sphere)/(initial velocity of core), must be at least 0.995, and is preferably from 1.000 to 1.008, and more preferably from 1.004 to 1.006. When this value is too large, the initial velocity of the finished ball may not conform to R&A standards, which may make the ball unacceptable as an official ball. On the other hand, when this value is too small, the spin rate on shots with a driver (W#1) may rise, as a result of which the intended distance may not be achieved.

Here, in the relationship in (8) above, the initial velocity of the core is preferably from 76.4 to 78.1 m/s, more preferably from 76.8 to 77.9 m/s, and even more preferably from 77.2 to 77.5 m/s. The initial velocity of the intermediate layer-encased sphere is preferably from 77.0 to 78.1 m/s, more preferably from 77.2 to 77.9 m/s, and even more preferably from 77.5 to 77.7 m/s. When these values are too large, the initial velocity of the finished ball may not conform to R&A standards, which may make the ball unacceptable as an official ball. On the other hand, when this value is too small, the finished ball has a low initial velocity and the initial velocity of the ball on shots with a W#1 may be low, as a result of which a good distance may not be achieved.

The initial velocity of the ball is typically at least 76.5 m/s, preferably at least 76.8 m/s, and more preferably from 77.0 to 77.7 m/s. At an initial velocity in excess of 77.724 m/s, the ball does not conform to R&A standards, making it unacceptable as an official ball. On the other hand, when this value is too small, the initial velocity on shots with a driver (W#1) may decrease, as a result of which a good distance may not be obtained. The initial velocity of the ball in relation to the initial velocity of the intermediate layer-encased sphere and the initial velocity of the core preferably satisfies the following condition:

(initial velocity of ball)<(initial velocity of core)<(initial velocity of intermediate layer-encased sphere).

The initial velocities of the core, intermediate layer-encased sphere and ball can be measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. In such a case, the core and intermediate layer-encased sphere can be tested in a chamber at a room temperature of 23±2° C. after being held isothermally in a 23±1° C. environment for at least 3 hours.

Numerous dimples may be formed on the cover (outermost layer). The number of dimples arranged on the cover surface, although not particularly limited, is preferably at least 280, more preferably at least 300, and even more preferably at least 320, with the upper limit being preferably not more than 360, more preferably not more than 350, and even more preferably not more than 340. When the number of dimples is larger than this range, the ball trajectory becomes lower, as a result of which the distance may decrease. On the other hand, when the number of dimples is too small, the ball trajectory becomes higher, as a result of which a good distance may not be achieved.

The dimple shapes that are used may be of one type or a combination of two or more types selected from among circular shapes, various polygonal shapes, dewdrop shapes and oval shapes. When circular dimples are used, the dimple diameter may be set to at least about 2.5 mm and up to about 6.5 mm, and the dimple depth may be set to at least 0.08 mm and up to about 0.30 mm.

In order to fully manifest the aerodynamic properties, it is desirable for the surface coverage ratio of dimples on the spherical surface of the golf ball, i.e., the ratio SR of the sum of the individual dimple surface areas, each defined by the flat plane circumscribed by the edge of a dimple, with respect to the spherical surface area of the ball were it to have no dimples thereon, to be set to at least 60% and up to 90%. Also, to optimize the ball trajectory, it is desirable for the value Vo, defined as the spatial volume of the individual dimples below the flat plane circumscribed by the dimple edge, divided by the volume of the cylinder whose base is the flat plane and whose height is the maximum depth of the dimple from the base, to be set to at least 0.35 and up to 0.80. Moreover, it is preferable for the ratio VR of the sum of the spatial volumes of the individual dimples, each formed below the flat plane circumscribed by the edge of a dimple, with respect to the volume of the ball sphere were the ball surface to have no dimples thereon, to be set to at least 0.6% and up to 1.0%. Outside of the above ranges in these respective values, the resulting trajectory may not enable a good distance to be obtained, and so the ball may fail to travel a fully satisfactory distance.

The multi-piece solid golf ball of the invention can be made to conform to the Rules of Golf for play. Specifically, the inventive ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm and is not more than 42.80 mm, and to a weight which is preferably from 45.0 to 45.93 g.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 5 Comparative Examples 1 to 6 Formation of Core

Solid cores for the respective Examples of the invention and Comparative Examples were produced by preparing the rubber compositions shown in Table 1 below, then molding and vulcanizing the compositions under the vulcanization conditions shown in the same table.

TABLE 1 Core formulations Example Comparative Example (pbw) 1 2 3 4 5 1 2 3 4 5 6 Polybutadiene A 80 80 100 100 100 80 80 80 80 80 80 Polybutadiene B 20 20 20 20 20 20 20 20 Zinc acrylate 27.5 26.5 28.0 27.5 27.4 33.5 36.5 27.5 27.5 27.5 26.5 Organic peroxide (1) 1.0 1.0 0.6 0.6 0.6 1.0 1.0 1.0 1.0 1.0 1.0 Organic peroxide (2) 0.6 0.6 0.6 Water 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 Antioxidant 0.1 0.1 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 Barium sulfate 19.7 20.2 21.6 22.0 22.0 17.2 15.8 19.7 13.6 19.7 20.2 Zinc oxide 4 4 5 5 5 4 4 4 4 4 4 Zinc salt of 0.5 0.5 1 1 1 0.5 0.5 0.5 0.5 0.5 0.5 pentachlorothiophenol Vulcanization First Temp. 155 155 145 145 145 155 155 155 155 155 155 conditions stage (° C.) Time 15 15 30 30 30 15 15 15 15 15 15 (min) Second Temp. — — 170 170 170 — — — — — — stage (° C.) Time — — 10 10 7 — — — — — — (min)

Details on the ingredients shown in Table 1 are given below.

-   Polybutadiene A: Available under the trade name “BR 01” from JSR     Corporation -   Polybutadiene B: Available under the trade name “BR 51” from JSR     Corporation -   Zinc acrylate: Available from Nippon Shokubai Co., Ltd. -   Organic peroxide (1): Dicumyl peroxide, available under the trade     name “Percumyl D” from NOF Corporation -   Organic peroxide (2): A mixture of 1,1-di(t-butylperoxy)-cyclohexane     and silica, available under the trade name “Perhexa C-40” from NOF     Corporation -   Water: Distilled water, from Wako Pure Chemical Industries, Ltd. -   Antioxidant: 2,2′-Methylenebis(4-methyl-6-butylphenol), available     under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical     Industry Co., Ltd. -   Barium sulfate: Available under the trade name “Barico #300” from     Hakusui Tech -   Zinc oxide: Available under the trade name “Zinc Oxide Grade 3” from     Sakai Chemical Co., Ltd.

Formation of Intermediate Layer and Cover

An intermediate layer material formulated as shown in Table 2 was injected-molded over the core obtained above, thereby giving an intermediate layer-encased sphere. Next, using the cover materials formulated as shown in Table 2, a cover (outermost layer) was injection-molded over the resulting intermediate layer-encased sphere, thereby producing a golf ball having an intermediate layer and a cover (outermost layer) over the core. Although not shown in the diagram, a common dimple pattern was formed on the surface of the ball in each of the Examples of the invention and the Comparative Examples.

TABLE 2 Resin materials (pbw) I II III IV T-8295 75 100 T-8290 25 Himilan ® 1706 37.5 Himilan ® 1557 37.5 AN4319 20 25 AN4221C 80 Hytrel ® 4001 11 11 Titanium oxide 3.9 3.9 2.0 Polyethylene wax 1.2 1.2 Isocyanate compound 7.5 7.5 Magnesium stearate 60 Calcium hydroxide 1.5 Magnesium oxide 1 Polytail H 8

Details on the materials shown in Table 2 are as follows.

-   T-8295, T-8290: MDI-PTMG type thermoplastic polyurethanes available     from DIC Bayer Polymer under the trademark Pandex. -   Himilan® 1706, Himilan® 1557:     -   Ionomers available from DuPont-Mitsui Polychemicals Co., Ltd. -   AN4319, AN4221C: An unneutralized ethylene-methacrylic acid-acrylic     acid ester terpolymer and an unneutralized ethylene-acrylic acid     copolymer (Nucrel®, from DuPont-Mitsui Polychemicals Co., Ltd.) -   Hytrel 4001: A polyester elastomer available from DuPont-Toray Co.,     Ltd. -   Polyethylene wax: “Sanwax 161P” from Sanyo Chemical Industries, Ltd. -   Isocyanate compound: 4,4′-Diphenylmethane diisocyanate -   Magnesium stearate: “Magnesium Stearate G” from NOF Corporation -   Calcium hydroxide: “Calcium Hydroxide CLS-B” from Shiraishi Calcium     Kaisha, Ltd. -   Magnesium oxide: “Kyowamag MF 150” from Kyowa Chemical Industry Co.,     Ltd. -   Polytail H: Available from Mitsubishi Chemical Corporation

For each of the resulting golf balls, properties such as the core hardness profile, thicknesses and material hardnesses of the respective layers, and the surface hardnesses of various layer-encased spheres were evaluated by the methods described below. The results are shown in Table 3 (Working Examples) and Table 4 (Comparative Examples).

Core Hardness Profile

The indenter of a durometer was set so as to be substantially perpendicular to the spherical surface of the core, and the core surface hardness in terms of JIS-C hardness was measured as specified in JIS K6301-1975.

To obtain the cross-sectional hardnesses at the center and other specific positions of the core, the core was hemispherically cut so as form a planar cross-section, and measurements were carried out by pressing the indenter of a durometer perpendicularly against the cross-section at the measurement positions. These hardnesses are indicated as JIS-C hardness values.

The Shore D hardness at the core surface was measured with a type D durometer in accordance with ASTM D2240-95.

Diameter of Core or Intermediate Layer-Encased Sphere

The diameters at five random places on the surface were measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single core or intermediate layer-encased sphere, the average diameter for five measured cores or intermediate layer-encased spheres was determined.

Ball Diameter

The diameters at five random dimple-free areas on the surface of a ball were measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single ball, the average diameter for five measured balls was determined.

Deflection of Core, Intermediate Layer-Encased Sphere and Ball

A core, intermediate layer-encased sphere or ball was placed on a hard plate and the amount of deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured. The amount of deflection here refers in each case to the measured value obtained after holding the test specimen isothermally at 23.9° C. In the table, letting A be the core deflection, B be the deflection by the intermediate layer-encased sphere and C be the ball deflection, differences between the deflections (the A−B value and the A−C value) and the sum of the deflections (the A+B+C value) were calculated.

Initial Velocities of Core, Intermediate Layer-Encased Sphere and Ball

The initial velocities were measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The cores, intermediate layer-encased spheres and balls (referred to below as “spherical test specimens”) were held isothermally in a 23.9±1° C. environment for at least 3 hours, and then tested in a chamber at a room temperature of 23.9±2° C. Each spherical test specimen was hit using a 250-pound (113.4 kg) head (striking mass) at an impact velocity of 143.8 ft/s (43.83 m/s). One dozen spherical test specimens were each hit four times. The time taken for the test specimen to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity (m/s). This cycle was carried out over a period of about 15 minutes.

Material Hardnesses of Intermediate Layer and Cover (Shore D Hardnesses)

The intermediate layer and cover-forming resin materials were molded into sheets having a thickness of 2 mm and left to stand for at least two weeks, following which the Shore D hardnesses were measured in accordance with ASTM D2240-95.

Surface Hardnesses of Intermediate Layer and Ball (Shore D Hardnesses)

Measurements were taken by pressing the durometer indenter perpendicularly against the surface of the intermediate layer-encased sphere or ball (i.e., the surface of the cover). The surface hardness of the ball (cover) is the measured value obtained at dimple-free places (lands) on the ball surface. The Shore D hardnesses were measured with a type D durometer in accordance with ASTM D2240-95.

TABLE 3 Example 1 2 3 4 5 Construction 3-piece 3-piece 3-piece 3-piece 3-piece Core Diameter (mm) 37.7 37.7 37.7 37.7 37.7 Weight (g) 32.9 32.9 32.9 32.9 32.9 Specific gravity 1.173 1.173 1.173 1.173 1.173 Deflection A (mm) 4.60 4.79 4.60 4.75 4.78 Hardness Surface hardness (Cs) 74 73 68 65 63 profile (JIS-C) Hardness at position 71 70 64 62 62 15 mm from center (C15) Hardness at position 61 61 63 61 61 10 mm from center (C10) Hardness at position 58 57 63 61 61 5 mm from center (C5) Center hardness (Cc) 55 54 63 61 61 Cs − C15 4 3 4 3 1 C15 − C10 9 9 1 1 1 C10 − C5 3 4 0 0 0 C5 − Cc 3 3 0 0 0 Surface − Center (Cs − Cc) 19 19 5 4 2 C10 − Cc 6 7 0 0 0 Cs − C10 13 12 5 4 2 (Cs − C10)/(C10 − Cc) 2.3 1.7 — — — Surface hardness (Shore D) 48 47 44 41 40 Initial velocity (m/s) 77.3 77.3 77.4 77.4 77.4 Intermediate Material I I I I I layer Thickness (mm) 1.65 1.65 1.65 1.65 1.65 Specific gravity 0.96 0.96 0.96 0.96 0.96 Material hardness (Shore D) 55 55 55 55 55 Intermediate Diameter (mm) 41.0 41.0 41.0 41.0 41.0 layer- Weight (g) 40.6 40.6 40.6 40.6 40.6 encased Deflection B (mm) 3.93 4.08 3.90 4.07 4.10 sphere Surface hardness (Shore D) 63 63 63 63 63 Initial velocity (m/s) 77.6 77.6 77.7 77.7 77.7 Intermediate layer surface hardness − 15 16 19 22 23 Core surface hardness Deflection difference (A − B) 0.67 0.71 0.70 0.68 0.68 Cover Material II II II II II Thickness (mm) 0.85 0.85 0.85 0.85 0.85 Specific gravity 1.15 1.15 1.15 1.15 1.15 Material hardness (Shore D) 53 53 53 53 53 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.5 45.5 45.5 45.5 45.5 Deflection C (mm) 3.50 3.59 3.48 3.57 3.59 Surface hardness (Shore D) 61 61 61 61 61 Initial velocity (m/s) 77.1 77.0 77.1 77.0 77.0 Core surface hardness − −13 −14 −17 −20 −21 Ball surface hardness (Shore D) Ball surface hardness − Intermediate −2 −2 −2 −2 −2 layer surface hardness (Shore D) Intermediate layer thickness − 0.80 0.80 0.80 0.80 0.80 Cover thickness (mm) Deflection difference (A − C) 1.10 1.20 1.12 1.18 1.19 Sum of deflections (A + B + C) 12.02 12.46 11.98 12.39 12.47 Initial velocity of intermediate layer- 1.004 1.005 1.004 1.004 1.004 encased sphere/Core initial velocity Initial velocity of intermediate layer- 0.31 0.38 0.32 0.32 0.32 encased sphere − Core initial velocity (m/s) Cover thickness + Intermediate layer 2.50 2.50 2.50 2.50 2.50 thickness (mm)

TABLE 4 Comparative Example 1 2 3 4 5 6 Construction 3-piece 3-piece 3-piece 3-piece 3-piece 3-piece Core Diameter (mm) 37.7 37.7 37.7 39.5 37.7 37.7 Weight (g) 32.9 32.9 32.9 36.8 32.9 32.9 Specific gravity 1.173 1.173 1.173 1.140 1.173 1.173 Deflection A (mm) 3.49 2.95 4.60 4.60 4.60 4.79 Hardness Surface hardness (Cs) 83 87 74 74 74 73 profile (JIS-C) Hardness at position 77 81 71 71 71 70 15 mm from center (C15) Hardness at position 65 68 61 61 61 61 10 mm from center (C10) Hardness at position 64 68 58 58 58 57 5 mm from center (C5) Center hardness (Cc) 61 62 58 58 58 54 Cs − C15 6 6 4 4 4 3 C15 − C10 12 14 9 9 9 9 C10 − C5 1 0 3 3 3 4 C5 − Cc 3 6 3 3 3 3 Surface − Center (Cs − Cc) 22 25 19 19 19 19 C10 − Cc 4 5 6 6 6 7 Cs − C10 18 19 13 13 13 12 (Cs − C10)/(C10 − Cc) 4.9 3.6 2.3 2.3 2.3 1.7 Surface hardness (Shore D) 55 58 48 48 48 47 Initial velocity (m/s) 77.7 77.7 77.3 77.2 77.3 77.3 Intermediate Material I I I I IV IV layer Thickness (mm) 1.65 1.65 1.65 0.75 1.65 1.65 Specific gravity 0.96 0.96 0.96 0.96 0.96 0.96 Material hardness (Shore D) 55 55 55 55 55 55 Intermediate Diameter (mm) 41.0 41.0 41.0 41.0 41.0 41.0 layer- Weight (g) 40.6 40.6 40.6 40.6 40.6 40.6 encased Deflection B (mm) 3.02 2.58 3.93 4.13 3.91 4.06 sphere Surface hardness (Shore D) 63 63 63 63 63 63 Initial velocity (m/s) 77.7 77.8 77.6 77.6 76.7 76.6 Intermediate layer surface hardness − 8 5 15 15 15 16 Core surface hardness Deflection difference (A − B) 0.46 0.37 0.67 0.47 0.69 0.73 Cover Material II II III II II II Thickness (mm) 0.85 0.85 0.85 0.85 0.85 0.85 Specific gravity 1.15 1.15 1.15 1.15 1.15 1.15 Material hardness (Shore D) 53 53 56 53 53 53 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.5 45.5 45.5 45.5 45.5 45.5 Deflection C (mm) 2.75 2.35 3.40 3.57 3.48 3.57 Surface hardness (Shore D) 61 61 64 61 61 61 Initial velocity (m/s) 77.1 77.1 77.0 77.0 76.4 76.3 Core surface hardness − −6 −3 −16 −13 −13 −14 Ball surface hardness (Shore D) Ball surface hardness − Intermediate −2 −2 1 −2 −2 −2 layer surface hardness (Shore D) Intermediate layer thickness − 0.80 0.80 0.80 −0.10 0.80 0.80 Cover thickness (mm) Deflection difference (A − C) 0.73 0.60 1.20 1.03 1.12 1.22 Sum of deflections (A + B + C) 9.26 7.88 11.92 12.30 11.99 12.42 Initial velocity of intermediate layer- 1.001 1.001 1.004 1.005 0.992 0.992 encased sphere/Core initial velocity Initial velocity of intermediate layer- 0.08 0.07 0.31 0.37 −0.64 −0.64 encased sphere − Core initial velocity (m/s) Cover thickness + Intermediate layer 2.50 2.50 2.50 1.60 2.50 2.50 thickness (mm)

In addition, the flight performance (W#1), spin performance on approach shots, feel and durability to cracking of the golf balls obtained in the respective Examples of the invention and the Comparative Examples were evaluated according to the criteria indicated below. The results are shown in Table 5.

Flight Performance (W#1 Shots)

A W#1 club (driver) was mounted on a golf swing robot, and the distance traveled by the ball when struck at a head speed (HS) of 35 m/s was measured and rated according to the criteria shown below. The club was a PHYZ III driver (2011 model; loft angle, 11.5°) manufactured by Bridgestone Sports Co., Ltd. The spin rate was measured, using an apparatus for measuring the initial conditions, immediately after the ball was similarly struck.

Rating Criteria:

-   -   Good: Total distance was 152.0 m or more     -   Fair: Total distance was at least 151.0 m but less than 152.0 m     -   NG: Total distance was less than 151.0 m

Spin Performance on Approach Shots

A sand wedge was mounted on a golf swing robot, and the spin rate of the ball when hit at a head speed (HS) of 20 m/s was rated according to the following criteria.

Rating Criteria:

-   -   Good: Spin rate was 5,600 rpm or more     -   NG: Spin rate was less than 5,600 rpm

Feel

Sensory evaluations were carried out when the balls were hit with a driver (W#1) by amateur golfers having head speeds of 30 to 40 m/s. The feel of the ball was rated according to the following criteria.

Rating Criteria:

-   -   Good: Six or more out of ten golfers rated the feel as good     -   Fair: Three to five out of ten golfers rated the feel as good     -   NG: Two or fewer out of ten golfers rated the feel as good

Here, a “good feel” refers to a feel at impact that is appropriately soft.

Durability to Cracking

The same type of driver (W#1) as that used in the flight performance evaluation was mounted on a golf swing robot and the ball was repeatedly struck at a head speed of 45 m/s. For the ball in each Example, a loss of durability was judged to have occurred when the initial velocity of the ball fell to or below 97% of the average initial velocity for the first ten shots. The average value for three measured golf balls (N=3) was used as the basis for evaluation in each Example. The durability indexes for the balls in the respective Examples were calculated relative to an arbitrary index of 100 for the number of shots taken with the ball in Example 2, and the durability was rated according to the following criteria.

Rating Criteria:

-   -   Excellent: Durability index was 110 or more     -   Good: Durability index was at least 95 but less than 110     -   NG: Durability index was less than 95

TABLE 5 Example Comparative Example 1 2 3 4 5 1 2 3 4 5 6 Flight W#1 Spin rate 3,152 3,073 3,289 3,208 3,199 3,484 3,577 3,103 3,137 3,265 3,177 HS, 35 m/s (rpm) Total 153.5 154.0 151.2 151.7 151.5 149.7 148.5 153.8 152.3 148.0 148.3 distance (m) Rating good good fair fair fair NG NG good good NG NG Performance Spin rate 5,654 5,618 5,659 5,622 5,607 5,817 5,852 5,506 5,595 5,659 5,623 on approach (rpm) shots Rating good good good good good good good NG good good good Feel Rating good good good good good fair NG good good good good Durability Rating good good Exc. Exc. Exc. good good good NG good good to cracking

In Comparative Example 1, the ball deflection, intermediate layer-encased sphere deflection and core deflection under specific loading were all small, meaning that each of these spheres was hard. As a result, the spin rate on full shots with a W#1 was high and a good distance was not obtained. Also, the ball had a hard feel at impact.

In Comparative Example 2, the ball deflection, intermediate layer-encased sphere deflection and core deflection under specific loading were all small, meaning that each of these spheres was hard. As a result, the spin rate on full shots with a W#1 was high and a good distance was not obtained. Also, the ball had a hard feel at impact.

In Comparative Example 3, the surface hardness of the ball was higher than the surface hardness of the intermediate layer-encased sphere. As a result, the spin performance on approach shots was poor.

In Comparative Example 4, the intermediate layer thickness was smaller than the cover thickness. As a result, the spin rate on full shots with a W#1 was high and a good distance was not obtained. Also, the durability on repeated impact was poor.

In Comparative Example 5, the (initial velocity of intermediate layer-encased sphere)/(core initial velocity) value was smaller than 0.995, as a result of which the ball had a low initial velocity. In addition, the spin rate on full shots rose and so the intended distance was not achieved.

In Comparative Example 6, the (initial velocity of intermediate layer-encased sphere)/(core initial velocity) value was smaller than 0.995, as a result of which the ball had a low initial velocity. In addition, the spin rate on full shots rose and so the intended distance was not achieved.

Japanese Patent Application No. 2015-118026 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A multi-piece solid golf comprising a core, a cover and an intermediate layer therebetween, wherein a sphere comprising the core and the intermediate layer which peripherally encases the core (intermediate layer-encased sphere) and the ball have respective surface hardnesses, expressed in terms of Shore D hardness, which satisfy the relationship: (Shore D hardness at ball surface)≦(Shore D hardness at surface of intermediate layer-encased sphere);  (1) the intermediate layer and the cover have respective thicknesses which satisfy the relationship: cover thickness≦intermediate layer thickness;  (2) the intermediate layer-encased sphere and the core have respective initial velocities which satisfy the relationship: (initial velocity of intermediate layer-encased sphere)/(initial velocity of core)≧0.995; and  (3) the core, the intermediate layer-encased sphere and the ball, when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), have respective deflections (mm) A, B and C which satisfy the condition: A+B+C≧11.5,  (4) with the proviso that A≧4.0 and C≧3.2.
 2. The golf ball of claim 1 which, in formula (3), satisfies the condition: (initial velocity of intermediate layer-encased sphere)/(initial velocity of core)≧1.004.
 3. The golf ball of claim 1 which, in formula (4), satisfies the condition (A−C)≧0.9.
 4. The golf ball of claim 1 wherein the core has a hardness profile which, expressed in terms of JIS-C hardness, satisfies conditions (i) to (vi) below, wherein Cc is the JIS-C hardness at a center of the core, C5 is the JIS-C hardness at a position 5 mm from the core center, C10 is the JIS-C hardness at a position 10 mm from the core center, C15 is the JIS-C hardness at a position 15 mm from the core center, and Cs is the JIS-C hardness at a surface of the core: 18≦Cs−Cc,  (i) 0<C10−Cc≦10,  (ii) C10−Cc<Cs−C10,  (iii) 10<Cs−C10,  (iv) Cs≧68, and  (v) Cc≧48  (vi).
 5. The golf ball of claim 1 which further satisfies condition (iii-a) below: (Cs−C10)/(C10−Cc)≧1.0  (iii-a).
 6. The golf ball of claim 1 which further satisfies condition (vii) below: Cs−Cc<10  (vii).
 7. The golf ball of claim 1, wherein the intermediate layer is formed of a material obtained by blending as essential components: 100 parts by weight of a resin component comprising, in admixture, a base resin of (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer mixed with (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio between 100:0 and 0:100, and (e) a non-ionomeric thermoplastic elastomer in a weight ratio between 100:0 and 50:50; (c) 5 to 80 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1,500; and (d) 0.1 to 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in the base resin and component (c).
 8. The golf ball of claim 1, wherein the ball, the intermediate layer-encased sphere and the core have respective initial velocities which satisfy the relationship: initial velocity of ball<initial velocity of core<initial velocity of envelope layer-encased sphere  (5).
 9. The golf ball of claim 1 which, in formula (1), satisfies the condition: (Shore D hardness at surface of intermediate layer-encased sphere)≧(Shore D hardness at core surface)  (1′). 